使用Jscience计算n个单位的价格（提供单位价格和数量）

Question

I am looking for a better way to compute using Jscience but the generic pattern is much tougher to get a clear solution. I need to compute price for n units provided the unit price for a unit quantity is defined. like,

    Measure<Double, ? extends Quantity> unitQuantity = Measure.valueOf(1.0,
            Unit.valueOf("kg"));
    Amount<Money> unitPrice = Amount.valueOf(150.0, USD);
    Measure<Double, ? extends Quantity> quantity = Measure.valueOf(500.0,
            MILLI(Unit.valueOf("kg")));
    Amount<Money> amount = unitPrice.times(quantity.to(unitQuantity.getUnit())
                       .getValue() / unitQuantity.getValue());

For the above code I am getting this error:

The method to(Unit<capture#7-of ? extends Quantity>) in the type 
Measure<Double,capture#7-of ? extends Quantity> is not applicable for 
the arguments (Unit<capture#8-of ? extends Quantity>)

I read about generics at Java Tutorials and tried the following, but still no clear solution:

    Amount<Money> amount = compute(unitPrice,unitQuantity,quantity)

    private <T extends Quantity> Amount<Money> compute(Amount<Money> unitPrice,
        Measure<Double, T> unitQuantity, Measure<Double, T> quantity) {
    return unitPrice.times(quantity.to(unitQuantity.getUnit()).getValue()
            / unitQuantity.getValue());
}

Now getting this error:

   The method compute(Amount<Money>, Measure<Double,T>, Measure<Double,T>) 
   in the type JscienceEx is not applicable for the arguments (Amount<Money>, 
   Measure<Double,capture#7-of ? extends Quantity>, Measure<Double,capture#8-of ?    
   extends Quantity>)

Answer 1

The Unit.valueOf("kg") factory method may be convenient in some cases. But in this case, it does not retain enough information. It only returns the unit of "some" Quantity - hence the ? extends Quantity ? extends Quantity in the return type. But you don't know whether this is quantity is, for example, Mass or Power . And as long as you don't know this, you can not say whether the call to quantity.to(unitQuantity.getUnit()) is valid.

In this case, you can solve this by using a new BaseUnit<Mass>("kg") instead of the Unit.valueOf("kg") :

import static javax.measure.unit.SI.MILLI;

import javax.measure.Measure;
import javax.measure.quantity.Mass;
import javax.measure.unit.BaseUnit;

import org.jscience.economics.money.Currency;
import org.jscience.economics.money.Money;
import org.jscience.physics.amount.Amount;

public class JScienceUnits
{
    public static void main(String[] args)
    {
        BaseUnit<Mass> kg = new BaseUnit<Mass>("kg");
        Measure<Double, Mass> unitQuantity = Measure.valueOf(1.0, kg);
        Amount<Money> unitPrice = Amount.valueOf(150.0, Currency.USD);
        Measure<Double, Mass> quantity = Measure.valueOf(500.0, MILLI(kg));
        Amount<Money> amount = unitPrice.times(
            quantity.to(unitQuantity.getUnit()).getValue() / 
            unitQuantity.getValue());

        System.out.println(
            "Money for "+quantity+
            " with unit price "+unitPrice+
            " is "+amount);
    }
}

Answer 2

Found two approaches. ConversionException get thrown when conversion fails.

@SuppressWarnings("unchecked")
public static <U extends Quantity, V extends Quantity> Amount<Money> computeAmount(
        Measure<Double, U> unitQuantity, Amount<Money> unitRate,
        Measure<Double, V> netQuantity) {
    return unitRate.times(netQuantity.to((Unit<V>) unitQuantity.getUnit())
            .getValue() / unitQuantity.getValue());
}

-

public static <U extends Quantity, V extends Quantity> Amount<Money> computeAmount(
        Measure<Double, U> unitQuantity, Amount<Money> unitRate,
        Measure<Double, V> netQuantity) {
    UnitConverter toUnitQuantityUnit = netQuantity.getUnit()
            .getConverterTo(unitQuantity.getUnit());
    return unitRate
            .times(toUnitQuantityUnit.convert(netQuantity.getValue())
                    / unitQuantity.getValue());
}